The present invention is generally directed to testing the mechanical properties of films and coatings, and more particularly directed testing scratch and mar resistance of coatings and to a testing apparatus used therein.
In principle, marring process can be caused by contact of a coated surface with a moving solid body, which induces stresses in the coated surface. One example of the marring process can be seen in coatings on automotive bodies, which are typically exposed to damage by abrasive elements, such as dust, dirt, surface scuffing during a car wash and weathering action. The marring of the coated surface results in loss of its esthetic appearance. The scratching or marring of a coated surface is especially undesirable in pigmented or clear coatings having high gloss. The scratch and mar resistance of a coating depends upon the physical properties, such as yield stress, toughness, Young""s modulus and hardness of the coating composition. The foregoing physical properties are greatly affected by the properties, such as glass transition temperature, and chemical structure of the polymers included in the coating compositions. Thus, the measurement of scratch and mar resistance of a coating becomes very important in selecting the components, such as polymers, used in coating compositions. For example, by comparing the mar resistance of a coating composition containing one type of polymer against that containing a different polymer may be used to decide which polymer is better suited for providing a coating with optimal long term gloss and other physical properties.
One approach provides for rubbing a sand paper of a well defined structure in a prescribed fashion against a coated surface for inducing damage on the coated surface. Multiple contact points on the sand particles of the sand paper induce the damage, which is then visually quantified, typically on a scale of 0 to 10, wherein the number 10 represents no damage and 0 represents total damage. Alternatively, the damage induced on one coating is visually compared against the damage on a coating from a different coating composition under the same test conditions to determine which coating has lesser damage. Nonetheless, due to the subjective nature of any visual observation, which tends to vary from person to person, the foregoing tests are not sufficiently objective. Furthermore, the sand papers, though standardized, tend to have different grain structures, which can greatly affect the damage produced on the coating. It is also very difficult to quantitatively measure the damage produced by the multiple contact points of the sand particles on the sand paper. Additionally, digital pressure applied by tester during the prescribed rubbing action tends to vary. As a result, the damage resulting therefrom also tends to vary from one tester to the next. Thus, a need exists for a mar resistance testing apparatus that is less subjective and is more reproducible than the subjective non-reproducible testing procedures currently in use.
One apparatus in the literature is described by Wu in J. Mater. Res., Vol. 6, No. 2, pages 407 to 426, February 1991 (Materials Research Society). The apparatus defined in that article is not sufficiently robust for reproducible measurements on a large variety of coated substrates under conditions of high throughput.
The present invention is directed to an apparatus for measuring mar resistance of a test sample comprising:
means for indentor guiding, said means being mounted on a post of said apparatus comprise:
means for indentor driving having an indentor positioned therein, and means for sensing travel of said indentor towards and away from the surface of said test sample; and
means for directing test sample, said means being positioned on a base of said apparatus comprise:
holder means to secure said test sample thereon with the surface of said test sample in perpendicular relationship with said indentor, and staging means for traversing said test sample in a direction tangential to said indentor, such that when a tip of said indentor is simultaneously driven into said test sample, a scratch a produced on the surface of said film or surface.
The present invention is also directed to a method of measuring mar resistance of a test sample comprising;
securing said test sample in staging means of an apparatus;
positioning an indentor in a perpendicular relationship to the exposed surface of said test sample, such that a tip of said indentor is in contact with said surface of said test sample;
driving said tip of said indentor into the surface of said test sample at a set rate while simultaneously traversing said test sample in a direction tangential to said indentor at a set speed in a set direction for scratching said surface of said test sample to produce a scratch thereon.